invalid type internal size -1

I all,
I'm tryng to declare my email type.
I think I have to do it in this way:

create type email ( INPUT = email_in,
OUTPUT = email_out,
RECEIVE = email_recv,
SEND = email_send,
INTERNALLENGTH = VARIABLE,
PASSEDBYVALUE = f,
ALIGNMENT = int4 );

however I get:
ERROR: invalid type internal size -1

In the doc is written:

Base data types can be fixed-length, in which case internallength is a positive integer, or variable
length, indicated by setting internallength to VARIABLE. (Internally, this is represented by setting
typlen to -1.)

I'm doing this on Postgres 7.4.1

Am I missing something ?

Regards
Gaetano Mendola